Method of depressing the liquidus temperature of lanthanum aluminogermanate glasses, and the glasses produced

ABSTRACT

The liquidus temperature of a lanthanum aluminogermanate glass is depressed by incorporating a selected additive in the glass batch. This produces a family of lanthanum aluminogermanate glasses composed essentially, in percent by weight on an oxide basis, of 25-50 percent GeO2, 1.5-25 percent Al2O3, 15-40 percent La2O3 and 2.5-45 percent of one or more oxides selected from the group Nb2O5, Ta2O5, SrO, BaO, PbO, and ZnO. These glasses have a substantial transmission for radiation in the five to six micron wavelength range, are hard and abrasion resistant, and have moderate coefficients of thermal expansion and good glassworking properties. They are particularly adapted to use as windows in infrared detection systems.

Unite States Patent [1 1 Dumbaugh, Jr.

[ 1 Oct. 30, 1973 METHOD OF DEPRESSING THE LIQUIDUS TEMPERATURE OFLANTHANUM ALUMINOGERMANATE GLASSES, AND THE GLASSES PRODUCED [75]Inventor: William H. Dumbaugh, Jr., Painted Post, NY.

[52] U.S. Cl. 106/47 Q, 106/47 R, 250/833 H [51] Int. Cl. C03c 3/12,C030 3/24 [58] Field oi Search 106/47 R, 47 Q [56] References CitedUNlTED STATES PATENTS 4/1962 Milne et al 106/47 Q 10/1954 Blan 106/47 RPrimary Examiner-Leland A. Sebastian Attorney-Clarence R. Patty, Jr. andClinton S, .Ianes, Jr.

[57] ABSTRACT The liquidus temperature of a lanthanum aluminogermanateglass is depressed by incorporating a selected additive in the glassbatch. This produces a family of lanthanum aluminogermanate glassescomposed essentially, in percent by weight on an oxide basis, of 25-50percent GeO 15-25 percent A1 0 15-40 percent 1521 0 and 2.5-45 percentof one or more oxides selected from the group Nb 'O Ta O SrO, BaO, PbO,and ZnO. These glasses have a substantial transmission for radiation inthe five to six micron wavelength range, are hard and abrasionresistant, and have moderate coefficients of thermal expansion and goodglassworking properties. They are particularly adapted to use as windowsin infrared detection systems.

3 Claims, 1 Drawing Figure METHOD OF DEPRESSING THE LIQUIDUS TEMPERATUREOF LANTHANUM ALUMINOGERMANATE GLASSES, AND THE GLASSES PRODUCED Thisinvention relates to the modification of lanthanum aluminogermanateglasses to depress the glass liquidus temperature, and to the infraredtransmitting glasses thus produced.

There has been a continuing interest for many years in the developmentof glasses having improved transmission characteristics in the infraredportion of the spectrum. A primary concern has been extension of thetransmission cutoff point or line to longer wavelengths. In recentyears, the application of these glasses in missile systems hasintroduced additional requirements, in particular resistance to erosionby rain and other atmospheric particles and resistance to heat shock.

Initial needs in the field of infrared transmitting glasses were metwith .silicate glasses. US. Pat. No. 3,531,306 granted in my name onSept. 29, 1970, discloses such infrared transmitting silicate typeglasses. Characteristically, the transmission cutoff in these glassesoccurs within the wavelength range of 4 to 5 microns.

In recent years, a need has developed for glasses which transmit well atlonger wavelengths, in particular within the wavelength range of 5 to5.5 microns. The inability of silicate glasses to meet this need, andthe consequent development of a family of calcium aluminogermanateglasses, are described in US. Pat. No. 3,531,305 also granted in my nameon Sept. 29, 1970.

The infrared transmission chacteristics of the glasses described in thislatter patent are substantially improved over those of known silicateglasses. Nevertheless, there has been a continuing desire to furtherimprove and extend transmission, particularly within the 5 to 5.5 micronrange. Also, the unexpected abrasive effect of rain and otheratmospheric particles on missile glass components has created a need toprovide harder, more abrasion resistant glasses than the calciumaluminogermanate glasses. It is, of course, desirableito achieve theseimprovements while maintaining other properties such as moderatecoefficient of thermal expansion," resistance to devitrification, andotherwise good glassworking characteristics.

The problem of improved infrared transmission in glasses has also beenaddressed in US. Pat. No. 3,119,703 which was granted to Cleek et al. onJan. 28, 1964 and which discloses a family of BaO-TiO -GeO glasses. Thecoefficients of thermal expansion of these glasses are generally above80 X and therefore undesirably high. Also such glasses may not beadequately hard for certain needs.

A companion application, Ser. No. 137,855, filed of even date herewithin my name, discloses a family of infrared transmitting glassescontaining substantial amounts of tantala (Ta O and zinc oxide (ZnO).The invention of that application is based on my discoveries thatlanthanum germanate type glasses have excellent infrared transmissioncharacteristics, and that a combination of Ta O and ZnO will stabilize alanthanum germanate melt without seriously depressing this desirableinfrared transmission.

It has been found that alumina (A1 0 will also stabilize lanthanumgermanate melts and provide a family of lanthanum aluminogermanateglasses. 1 have found that the resulting glasses provide a substantialtransmission of infrared radiation in the 5 to 6 micron wavelengthrange. However, they tend to have very high liquidus temperatures in therange of l,300-l ,400C. Accordingly, while such a glass can be melted,it is extremely difficult to cool the melt from the molten state andsubject it to any kind of molding or shaping operation without severedevitrification occurring.

I have now discovered that this problem of high liquidus temperature,and consequent devitrification, can be solved by incorporating one ormore selected additives in the composition of the lanthanumaluminogermanate base glass.

The present invention then is based on my discoveries that lanthanumaluminogermanate glasses are effective infrared transmitting glasses,and that the devitrification tendencies of these glasses can becontrolled by incorporating in the glass one or more of a selected groupof oxide additives.

A basic purpose of the present invention is to stabilize lanthanumgermanate melts with alumina, and to depress the devitrificationtendencies of the resulting lanthanum aluminogermanate glasses. Afurther purpose is to provide glasses that possess good infraredtransmission and physical hardness characteristics. Another purpose isto provide glasses that combine these properties with a moderatecoefficient of thermal expansion and good glassworking characteristics.

The invention then resides in a method of depressing the liquidustemperature of a lanthanum aluminogermanate glass by adding to the batchfrom which the glass is melted materials which incorporate into theglass one or more oxides selected from the group consisting of Nb O Ta OSrO, BaO, PbO, and ZnO, in an amount totalling 2.545 percent of theglass composition. This produces an infrared transmitting glassconsisting essentially, in percent by weight as calculated on an oxidebasis from the glass batch, of 2550 percent (3e0 1.5-25 percentAl2O3154O percent La O and 2.545 percent of one or more oxides selectedfrom the groupconsisting of Nb O Ta O SrO, BaO, PbO, and ZnQ.-Incidental amounts of other oxides may be prese'nt' to the extent thatthey do not unduly influence the basic glass properties. In general,such other glass constituents serve no useful purpose and theirintentional addition is avoided except as needed in glass melting orminor physical property adjustment.

Germania (GeO is the primary glass forming oxide in the present glasses,and at least 25 percent is required for that purpose. The rather strongdevitrification tendencies of the present glasses are lessened withincreased GeO contents. However, infrared transmission, particularly thelocation of the cutoff in the five to six micron range, generallyimproves as the GeO content decreases. Therefore, the GeO level selectedfor a given glass will generally represent a compromise between infraredtransmission and glassworking properties. In no case should the Geo:level exceed 50 percent.

Frequently, during working of low germania content glasses,devitrification will occur, but will be restricted to the glass surface.In that case, it may be possible to remove this defect by grinding andpolishing the finished product. In this manner, germania contents in thelower part of the range may be employed.

At least 15 percent lanthana (La O is necessary to provide a hard,abrasion resistant glass. This component also improves the infraredtransmission and extends the cutoff within the five to six micron rangeso that a content ver 20 percent is preferred. Up to 40 percent of thisoxide may be present before melting problems, particularlydevitrification, become intolerable.

Other oxides, such as baria (BaO) and calcia (CaO), are known to improveinfrared transmission in germanate glasses. However, I have found that alantharia glass provides a significantly superior transmission withrespect to a corresponding calcia glass, while generally also providinga substantially lower liquidus temperature. Lanthana tends to increasethe coefficient of thermal expansion of a germanate glass, but anequivalent amount of BaO tends to impart a much greater increase.Further, lanthana improves the chemical stability of a glass.

Alumina (M 0 tends to stabilize, both chemically and physically, alanthanum germanate glass. At least 1.5 percent Al O must be present inthe glass composition for this purpose.

It has been found, however, that the resulting La O -Al O -GeO glasseshave very high liquidus values, generally in the range of l,300-l,400C.Therefore, such glasses have a very strong tendency to devitrify as theycool and/or during subsequent forming operations.

I have found that several oxides are capable of lowerproperties measuredon the glasses, including (1) percent transmittance measured atwavelengths of 5 microns (%T and 5.5 microns (%T on a two mm. thickglass sample in a Perkin-Elmer Model 221 Infrared Spectrophotometer, (2)thermal expansion coefficient (Exp.) in units/C. X 10" as measuredbetween and 300C, and (3) liquidus temperature (Liq.) in C.

Percent T Percent T ing the liquidus temperature markedly in theseglasses, 2222: if; g 22, 2 thereby inhibiting the devitrificationtendencies. These Exp 63.9 73.7 61.! 63.2 60.9 58.0 Liq 1277 I289 l288i279 include the oxides of niobium, tantalum, strontium, barium, leadand zinc. At least 2.5 percent of one or more of these oxides isrequired for this purpose. However, glass melting may become moredifficult with intent should not exceed 45 percent for this reason.

In general other glass constituents provide no benefits in accordancewith the invention, and may actually depress the desirablecharacteristics of the glasses. Accordingly, they are normally avoidedexcept as incidental impurities or as minor additives in amounts up to 5percent or so for such purposes as physical property adjustment andfining. Thus, titania (TiO has little adverse effect on expansion orliquidus and may aid in melting or viscosity adjustment. However, glassforming oxides other than 0e0 particularly boric oxide (B 0 may sharplydepress transmission and are avoided. i

In general, the present glasses may be melted in conventional mannerfrom ordinary glassmaking materials and worked in any customary manner.However,'it is usually desirable to employ relatively pure raw materialsin oxide form, and to melt the glasses in small, electrically heated,melting units.

The drying procedure described in US. Pat. No. 3,53l,27l granted in myname on Sept. 29, 1970 may be employed. Briefly, this comprises adding aglass component to the batch in chloride form and melting the batch witha dry gas passing over the melt.

The invention is further described and illustrated with reference to thefollowing table which sets forth, in percent by weight on an oxidebasis, the compositions of several glasses produced in accordance withthe invention. Also set forth, in the table are certain A half-poundglass batch, corresponding to each of the above compositions, wasprepared by weighing oxide batch materials to the nearest 0.1 gram andmixing thoroughly. The mixed batch was placed in a platinum crucible andmelted in an electrical resistance furnace for 2 hours at l,500C. Themelt was cast to form a 2 inch X 4% inch slab, and this was annealed bycooling slowly from about 760C. to room temperature.

The above melting procedure might be modified by including a smallamount of a chloride material in the glass batch. For example, the CaOcomponent of Example 4 might be introduced into the glass batch ascalcium chloride (CaCI Also, the melting procedure might be modified byblowing a dry gas, such as nitrogen, over the crucible during melting.in this manner, a dry glass would be produced in accordance with theteaching in my earlier patent.

By way of specifically illustrating a particular feature of the presentinvention, a glass having the composition of Example 5 in the tableabove is compared with a present commercial infrared transmitting glassdesignated A. The latter glass has a coefficient of thermal expansion of62.6 X lO"/C., a liquidus temperature of l,324C., and is composed of43.2 percent 0 02, 5.1 percent ZnO, 23.7 percent A1 0 13.8 percerit CaO,9.7 percent BaO, 4.4 percent Clf' A two (2) mm. thick sample of eachglass was prepared and percent transmission measurements at wavelengthsover the range of 2.0-6.0 microns were made on the spectrophotometeridentified above. The curves plotted from these data are shown in theappended drawing which is a graphical illustration wherein per- TABLE IIA B c Geo 44 45 45 A1203 30 1s 2s L3 25 25 25 Nb O 5 n 0 5 Liq 1350 1I96 1270 It will be noted that the substitution of 5 mole percent T21 0for A1 0 (compare glasses A and C) lowers the liquidus temperature by C.The substitution of 15 mole percent of additives in glass B has the evenmore dramatic effect of lowering the liquidus temperature by l50C. tobelow 1,200C.

I claim:

1. A method of depressing the liquidus temperature of a lanthanumaluminogermanate glass which includes adding to the batch from which theglass is melted materials which incorporate into the glass one or moreoxides selected from the group consisting of Nb O Ta O SrO, BaO, PbO andZnO in an amount totalling 2.5-45 percent of the glass composition.

2. An infrared transmitting glass consisting essentially, in percent byweight as calculated on an oxide basis from the glass batch, of 25-50percent 6e0 l.525 percent M 0 15-40 percent La O and 2.545 percent ofone or more oxides selected from the group consisting of Nb O Ta O SrO,BaO, PbO, and ZnO.

3. An infrared transmitting glass in accordance with claim 2 wherein theLa O content is at least 20 percent.

2. An infrared transmitting glass consisting essentially, in percent byweight as calculated on an oxide basis from the glass batch, of 25-50percent GeO2, 1.5-25 percent Al2O3, 15-40 percent La2O3 and 2.5-45percent of one or more oxides selected from the group consisting ofNb2O5, Ta2O5, SrO, BaO, PbO, and ZnO.
 3. An infrared transmitting glassin accordance with claim 2 wherein the La2O3 content is at least 20percent.